Up-hole reflectometry devices that interrogate passive mechanical transducers are increasingly used in geotechnical instrumentation. One of such a monitoring technique is the time domain reflectometry (TDR). This study utilizes the principle of TDR to devise a TDR extensometer using a waveguide with an impedance mismatch interface inside it. The movement of the impedance mismatch interface is coupled with the displacement of interest such that the time shift in the reflected signal becomes a measure of the displacement. The accuracy of the TDR extensometer is better than +/- 0.5 mm and is not affected by measuring range and cable length. It can be used for applications that require both high accuracy and large displacement range, and is suitable for long-term monitoring in a harsh environment where humidity and lightening surge may cause problems for electronic sensors. The devised extensometer can be combined with existing TDR transducers for groundwater pressure and shear deformation to form an integrated TDR monitoring system for slope stability.